The first checkpoint frame whose state hash differs from the session, or null if all matched. Expected after the game's code changes (a fix changes the state); before a change it means the headless run drifted from the browser (physics, Math.* differences between engines, wall-clock reads), and frames after it may not reproduce what the player saw.
notes
string[]
The recording's notes plus anything the replay noticed.
recordedError?
object
The error the session ended with, if any; a faithful replay throws it at the same frame.